when talking about characters/bytes, use "nul" and "nul-terminate" when talking about pointers, use "null pointer" or ".Dv NULL"
added following manpages (derived from FreeBSD), wcwidth(3) wcswidth(3) wcstol(3) wcstoll(3) wcstoimax(3) wcstoul(3) wcstoull(3) wcstoumax(3) and update doc/TODO.i18n.